Work machines are used in many industries, such as the agricultural, construction, and forestry related industries, and employ transmissions for driving the work machine to and from various work sites, and for operating the work machine in performing its designated tasks. The transmissions of such work machines typically have more than one forward gear and more than one reverse gear so as to allow a gear to be selected that is optimal for the task at hand. In order to select and switch between gears, transmissions employ a shift system that actuates one or more hydraulic clutches in order to engage the desired gear.
The transmission shift systems of present work machine transmissions are often complex, requiring the use of an electronic controller that accepts input from a shift device, such as a shift lever, a dial, etc. Such controllers use the input to control valves that operate the appropriate clutches in order to obtain the desired transmission output speed, i.e., first gear, second gear, third gear, etc, and high range or low range, and to obtain the desired transmission output direction, i.e., forward, neutral, and reverse. However, a failure of the controller or any component in the control path, such as a valve, may have unforeseen consequences, such as unintended motion of the work machine.
Once such a failure has taken place, diagnostics are required to ensure that the failed component is accurately determined, which can then be repaired or replaced. However, with such controller-based transmission shift systems, the failures are often “hidden,” in the sense that it may be difficult to determine which component in the control path has failed. Often, special diagnostic tools are required in order to determine what component has failed, making it difficult to diagnose and correct failures in the field, even where only a single component of the transmission shift system has failed.
Hence, it is desirable to have a transmission shift system wherein a single-point failure, i.e., the failure of a single component of the transmission shift system, does not result in unintended motion, and wherein the single-point failure may be readily diagnosed in the field by the operator of the work machine, without the use of special diagnostic tools.